Experimental Study of High Frequency Impulse Overvoltages at Sayano-Shushenskaya HPP

Abstract

High-frequency switching overvoltages with an oscillation frequency of up to several MHz are most dangerous for power transformers and cable bushings with XLPE insulation. A working group in charge for studying the influence of high-frequency impulse overvoltages on the operational reliability of power transformers has been established within the CIGRE framework. Experimental studies of high-frequency impulse overvoltages in the 500 and 15.75 kV networks at the Sayano-Shushenskaya HPP were carried out. A new digital recording system equipped with contactless capacitive sensors was used for measuring impulse overvoltages. By using such recording system, it became possible to obtain data on the overvoltages experienced simultaneously by cable bushings and power transformers on the 500 and 15.75 kV voltage sides. The study results have shown that the highest peak values of high-frequency overvoltages are induced by switching operations of disconnectors in the 500 kV GIS. The overvoltages emerging in the 500 kV network are transferred to the 15.75 kV voltage level via the capacitances between the 500/15.75 kV transformer windings. The resulting overvoltage level may be more than a factor of ten higher than the amplitude value of the nominal 15.75 kV voltage. An analysis model was developed in the ATP-EMTP software, which has been verified against the experimental data. The accomplished numerical analyses and experiments have shown that the use of RC circuits is the most efficient protection measure against high-frequency overvoltages.